Airborne Transient ElectroMagnetic (ATEM) methods for geophysical exploration are well known in the art. Such methods are appropriate for finding ore bodies in the ground, especially if large areas are to be explored in remote regions which are not easily accessible from the ground. Certain types of ore bodies are highly conductive compared with their surroundings and are therefore well suited to detection by electromagnetic systems.
New helicopter ATEM systems provide a lower noise platform which can be flown more slowly and at lower altitude than fixed wing aircraft systems and hence can gather higher resolution data. Helicopter systems have restraints such as high flight-time costs. To control such costs, it is advantageous to use smaller helicopters, but smaller helicopters have limited payload capacity. Thus, it is desirable for ATEM sensor systems to be low in weight and sufficiently small, so as to minimize aerodynamic drag. Induction magnetometers in such systems should also have low noise, high sensitivity and large bandwidths.
Most of the early geophysical literature concentrates on air cored induction magnetometers because their performance is easier to predict. Without the flux concentrating effects of a permeable core, these conventional induction magnetometers tend to be awkward to work with because of their size and weight. With the advent of new ferrites and alloys such as MuMetal®, more development of cored induction magnetometers has been undertaken. Cored induction magnetometers have been used in electromagnetic interference measurements, space exploration, extremely low frequency/very low frequency (ELF/VLF) communications and seismo-electric geophysics exploration. Although there is some discussion in the literature in these fields about the related coil design, their optimization parameters usually differ from the objectives of ATEM.
Prior work in geophysical exploration has used arrays of magnetometers in order to speed up data gathering. Magnetometer arrays have also found some use at higher frequencies in magnetic resonance research. Tri-axial magnetometer-accelerometers have been used as gradiometers for sensing magnetic anomalies from a moving platform. The type of sensors used in most of these applications are not of the inductive type and are not suited for use in ATEM.
A basic setup of a helicopter-borne ATEM geophysical exploration system is illustrated in FIG. 1. A helicopter 100 carries a horizontal transmitter coil 102 below the helicopter 100, and tows a pod 104 which houses an orthogonal induction magnetometer. The transmitter coil 102 transmits a pulsed primary magnetic field 106. The signal of interest is produced when the transmitter coil 102 makes the transition between the ON state to the OFF state. During this transition, the magnetic field 106 that was applied to a conductor such as an ore body 108 beneath the ground 110, is changing with time and thus a current and a secondary field 112 is created. The current in the ore body 108 usually flows around its perimeter which forms closed contours. After the primary field 106 is turned off, the current in the ore body will decay over time. The decay time will be a function of the ore body's resistivity and inductance. For ore bodies which are good conductors, where the resistance is low, the current will persist for a longer time and thus the secondary field 112 will decay slowly. The inductance is determined by the size of the ore body, thus the larger the ore body deposit, the longer the decay time. For poor conductors, where the resistance is high, the current will decrease rapidly and thus the secondary field 112 will decay much faster. This secondary field is what is measured by the orthogonal induction magnetometer housed in the pod 104.
The system described in FIG. 1 is an example of an induced pulse transient system wherein the primary field is off during the measurement of the secondary field. This type of system has an advantage over the continuous primary field type systems because it avoids the problem of trying to measure a very small secondary field in the presence of a strong continuous primary field. One source of noise in such an ATEM system is from vibration caused by the motion of the sensors relative to the Earth's magnetic field and the secondary field.
For this reason, directional magnetometers are used. These types of magnetometers only respond to flux which is directed along their sensitive axis. By using three orthogonal magnetometers, the orientation of the magnetic field can be resolved, providing important information to geoscientists. This also means that signal processing must be used in order to remove vibration noise.
Conventional induction magnetometers (CIM) have long been the sensor of choice for electromagnetic exploration. The design of the CIM is based on Faraday's law of electromagnetic induction which states that the electromotive force induced in a stationary closed circuit is proportional to the product of the number of turns and the negative rate of increase of the magnetic flux linking the circuit. A basic example of a prior art CIM is illustrated in FIG. 2. The CIM indicated generally at 200 consists of a permeable core 202, surrounded by a multi-turn coil 204 of orthogonal windings. The total weight of the CIM 200 is made up of the weight of the core 202 and the weight of the windings of the coil 204. In an ATEM system, it is preferable to minimize the weight and size 206, 208, 210 of the induction magnetometer components. The basic CIM has several limitations. The closely spaced orthogonally-wound windings create capacitance which causes the CIM to self resonate. This resonance limits the usable bandwidth.
In practice, CIMs are usually constructed with scramble-wound coils in order to limit the winding capacitance. This winding technique distributes the turns in a pseudo-random fashion. Layers are not completely filled before proceeding to the next one. Although the spacing introduced decreases the capacitance, it increases the length of wire required to make the same number of turns and thus increases the weight and the resistance of the winding.
An induction magnetometer can be made more sensitive by increasing the magnetic flux captured by the coil and by increasing the number of windings of the coil. Increasing the number of windings in the coil also increases the weight of the coil, which is not desirable for magnetometers used for ATEM. Increasing the magnetic flux captured by a coil can be done by using permeable cores to concentrate the magnetic flux. Larger diameter cores will also capture more flux, but such cores will also be heavier, which is not desirable for magnetometers used for ATEM.
Cores with higher permeability will also concentrate more magnetic flux. Finite length permeable rod cores have non-uniform flux along their length. As illustrated in FIG. 3, the variation of the flux within the core 202 is caused by flux lines 302, which do not all enter and exit from the ends of the core. Since the inductance of a CIM is related to the amount of flux which threads the coil, this flux variation has a significant impact on the observed inductance and on the induced voltage of a CIM. The inductance due to windings around a permeable core is therefore dependant on the location of the windings on the core. FIG. 4 illustrates a triangular shape flux weighting function which relates the location of a winding to the output current, and illustrates that windings at the center of a permeable core have more effect than windings near the ends of the core. Also, an induction magnetometer having a longer permeable core will have a higher inductance, and unfortunately, also higher weight and longer length, neither of which are desirable for magnetometers used for ATEM.
Calibration of conventional 3-axis induction magnetometers, also known as orthogonal conventional induction magnetometers (OCIM), usually requires the disassembly of the OCIM. The individual CIMs are brought to a test range where they are placed at a distance from each other. The support for the transmitter loop and the OCIM are checked for orthogonality and the distance between the transmitter and the CIM is measured accurately. This must be done for every CIM which forms the OCIM. This is labor intensive, and time consuming, thus it is generally only done once before every survey. If the system malfunctions during the survey, the reliability of the data is compromised and the survey will have to be re-flown, adding considerable cost.
Accordingly, an improved design of the induction magnetometer, having increased sensitivity, improved signal-to-noise ratio and higher bandwidth, remains highly desirable.
An improved arrangement for calibration of an induction magnetometer for airborne geophysical surveys is also highly desirable.